Using HI absorption observations in front of extended continuum sources it is
possible to study the small scale structure of the cold galactic HI.
Cas A with a total flux density of
2100 Jy and a diameter of 5 arcmin provides high sensitivity because of the
high surface brightness of the source. The source has been observed previously with
various interferometers in HI. The present observations have an angular resolution
of 30'' and a velocity resolution of 0.6 km s
. Previous observations of Cas A
were observed with lower resolution and sensitivity using the Caltech interferometer
(Greisen 1973)
as well as the WSRT. The latter observations of Cas A were done to measure
the HI Zeeman splitting (Schwarz et al. 1986). More recently
Bieging et al. (1991)
observed the HI absorption against Cas A with the VLA with high angular and
velocity resolution (7'' , 0.6 km s
); however the velocity range was
limited to the "Perseus feature'' ( -55 < v < -30 km s
); the local
velocity, v > -30 km s
("Orion''
arm) was not observed. Their results
show a wealth of fine structure.
The purpose of the present publication is to present high resolution images
of the entire galactic HI spectrum of Cas A. We concentrate
however our discussion on the local velocities (> -30 km s
).
Our results can be used also to compare with obervations of molecular lines (e.g. Wilson et al. 1993), recombination lines (Anantharamaiah et al. 1994), but also with optical, IR and X-ray (Keohane et al. 1996) images of Cas A. The latter authors have used the WSRT data to calibrate the soft X-ray absorption in the direction of Cas A.
Small scale structure with resolutions similar to the present one were also found toward other sources, such as 3C 58 (Roberts et al. 1994), W3 (van der Werf & Goss 1989), 3C 10 (Schwarz et al. 1995), Orion (Lockart & Goss 1978; van der Werf & Goss 1990), W49 (Lockart & Goss 1978).
Cas A is at galactic
longitude 112
and at latitude 
. The line of sight samples the galaxy
without velocity ambiguity in the second quadrant; Cas A is believed to lie on the far side of the
Perseus arm, which has a distance of 
kpc from the sun
(Read et al. 1995). At low
velocities the HI has typically a distance of 100 pc ( the thickness of the
cold HI layer); the angular resolution corresponds then to 0.014 pc. At
velocities of the Perseus arm ( -52 < v < -34 km s
) with a distance of
3 kpc the linear resolution is 0.5 pc.
The spin temperature could in principle be deduced from the comparison of absorption features seen at the edge of the source with HI emission just outside the source. But no attempts were made to image the emission outside the source because of the confusion by the 10 arcmin grating response and the low sensitivity of 18 K at the original resolution. Moreover the strong sidelobes of this extended source with varying absorption as a function of velocity introduces additional uncertainties. Although the direct determination of the spin temperature is excluded, we can provide upper limits from the velocity widths of the features. Knowing the angular size of the HI concentrations and their distance, estimates of the space density can be made for a given spin temperature.
The main part of the paper consists of the display of data-cube of optical depths at all velocities. First, we give series of "channel-images'' (= maps at constant velocity), then we show cuts through the cube at constant declinations and finally a series of individual spectra. In the last section we give a short discussion of some of the results.